I like the user's enhanced ability to experiment with choices -- being able to preview what things will look like before committing to the choice. Of course, if you aren't timid about using Undo you can almost do the same thing today, albeit less conveniently.
It was mentioned that all existing add-ins would work in Office 12, but what about those that made modifications to the menus? Or added buttons to toolbars? Or created entirely new toolbars? Won't there need to be some reprogramming to make this compatible?